A Comparative Assessment of Benthic Macroinvertebrate Secondary Production in Impounded and Free-Flowing River Systems

It has been well documented that a river’s natural flow regime is critical in maintaining geomorphological and biological integrity.  A common and pervasive means by which flow regimes are altered is by anthropogenic impoundment.  Many studies have investigated the impacts river impoundments have on specific habitats and the constituents of the communities within them.  However, an assessment of benthic macroinvertebrate production may provide insight into not only the physical effects of impoundment but also the ecological and trophic limitations inflicted upon affected rivers.  We are in the process of estimating habitat specific rates of benthic macroinvertebrate secondary production and comparing those rates between physically similar impounded and free-flowing river systems.  These rates are then applied to estimations of area for specific habitats, extrapolating a reach-wide rate of secondary production.

Our study compares secondary production by macroinvertebrates in two pairs of river systems in the Yakima River (WA) watershed.  River pairs have been selected to be similar in terms of drainage area, land use, vegetation, precipitation, slope, and parent rock type.  In order to isolate the effects of impoundment on secondary production through changes in habitat availability vs. production within specific habitats, we quantify the amount of shallow pool, riffle, and side-channel habitats present, including the relative surface area of those habitats for each system.  We also directly quantify aspects of the physical habitat likely to be important for macroinvertebrates and altered by impoundment, including substrate composition, depth and flow characteristics.  Each habitat is sampled semimonthly, and estimates of secondary production calculated with the size-frequency method.  The habitat specific rates of secondary production are assessed between treatments then used as a means of estimating a reach-wide rate of secondary production.